The Heroes actor died in August at age 36 and investigations have been ongoing into the circumstances surrounding her death in South Carolina.
And now, multiple law enforcement sources with direct knowledge of the case have told TMZ that Panettiere had a "longtime drug dealer" in Los Angeles, who provided her with several black-market prescription medications, and she had taken a number of them with her on her flight from California the day before she died.
Authorities are said to be working on the theory she took the oxycodone pill on the day she died but it was laced with the strong opioid.
Toxicology results are still pending, but the Drug Enforcement Administration in Los Angeles are piecing together the relationship between the Nashville star and her drug dealer in an attempt to trace the pill back to him.
It was previously revealed that Panettiere's on-off boyfriend, Brian Hickerson, had handed first responders a bag of her prescription medications when they arrived at the apartment they had been staying at when she died.
An autopsy previously found "no signs of trauma" that had contributed to the Heroes star's death.
A statement from Greenville County Coroner's Office read: "An autopsy was completed today. At autopsy no signs of trauma were discovered that would have contributed to the death.
"The cause and manner of death remain pending further investigation and the completion of additional studies. This investigation remains active and ongoing. No further details are available for release at this time."
The statement confirmed first responders had found the actor suffering a cardiac arrest when they arrived on the afternoon of August 16.
TMZ obtained dispatch audio of the call to emergency services, which appears to pick up officials discussing an overdose.
The short recording included one officer saying: "Responding. Overdose. CPR is in progress."
After receiving the emergency call, paramedics rushed to the apartment and tried to resuscitate Panettiere using "advanced cardiac life support".
But after an hour, the Ice Princess star - who had 11-year-old daughter Kaya with former partner Wladimir Klitschko - was pronounced dead.
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